A magnetic tape is composed of such a base layer consisting of PET (polyethylene terephthalate), a magnetic layer formed on one surface of the base layer, a back layer formed on the other surface of the base layer, and the like. Such the magnetic tape is required to show predetermined stable performance when data is recorded in or reproduced from it.
For example, during manufacturing the magnetic tape, if it is damaged and foreign particles floating in a manufacturing factory adhere to its surfaces, a recording magnetic force and reproducing output are lowered due to the damage and foreign particles in recording and reproducing, whereby an error so called a “dropout” that data cannot be correctly recorded or reproduced occurs.
Therefore, it is necessary to accurately grasp surface (surfaces of magnetic layer and back layer) states of the magnetic tape in order to manufacture a high quality magnetic tape which can show the predetermined stable performance. So conventionally, damage, grooves, and the like formed on the surfaces of the magnetic tape are optically inspected. For example, as such an inspection method, “Method and Apparatus for Inspecting Surface of Magnetic Recording Medium” is described in FIG. 1 of Japan patent laid open publication 8-233560.
“Method and Apparatus for Inspecting Surface of Magnetic Recording Medium” described in the above patent is intended to improve inspection accuracy by contriving an equipment arrangement of a light emitting device, camera, and the like, and it is described in “the method and apparatus” that when emitting linear detection lights on the magnetic layer surface of the magnetic tape and discriminating presence or absence of surface defects by receiving reflection lights of the detection lights by a CCD camera, an output level of system noise included in the detection lights can be decreased by making angles of emitting/receiving lights within a specific range and furthermore making a position of light axes of incident lights deviate from a reflection point.
However, with magnetic recording highly being densified these years, although the inspection accuracy of surface strain of the magnetic tape is furthermore required to be improved, there exists a problem that “Method and Apparatus for Inspecting Surface of Magnetic Recording Medium” described in the patent cannot accurately detect the surface strain of the magnetic tape.